1. Field of the Invention
This invention relates to a method of manufacturing bi-metallic tubing by the successive steps of explosive bonding, hot extrusion and co-extrusion. The invention specifically relates to such a method for use in the manufacture of bi-metallic tubing wherein the inner wall of the tube is made from a corrosion resistant material.
2. Description of the Prior Art
There is a considerable requirement for bi-metallic specifically for use in drill pipe and distribution pipeline in the oil and gas industries. Conventionally, such tubes consist of a principal metallic tube made from a first metal to which is bonded a metallic layer made from a second metal, the second metal being a more expensive, corrosion, resistant material. The use of such tubing leads to the reduction of costs by removing the need to manufacture corrosion resistant tubing which would be more expensive were it to consist entirely of the more expensive material and be made in a wall thickness which is sufficient to meet normal pressure requirements. The corrosion resistant layer may be within the tube bore, on the outer surface, or both. In normal uses, however, the corrosion resistant layer will be on the inner surface of the tube through which the corrosive material passes.
Co-extrusion is the predominant method of manufacture of this type of tube. This method consists of placing a first tube of corrosion resistant metal into the bore of a second and much thicker walled tube of less expensive metal, usually steel. The external diameter of the outer tube will be a close fit into the container of an extrusion press and will typically be some 300 mm to 400 mm in dimension. The internal bore will vary and be dependent upon the press mandrel being used to produce the specific bore of the extruded `shell` to be formed. The length of the loose composite, i.e. first and second tubes, will be that required for the container of the extrusion press, and the proportional wall thickness of the two metallic tubes will be identical to that desired in the final tube to be produced.
The interface of the tubes is sealed prior to heating the composite billet which is then extruded. On extrusion, the two metals become bonded at the interface.
The principal limitation of this process is that, to be successful, the two metals being used must be compatible. Compatibility is usually associated with a small differential in the mechanical properties and atomic spacing of the two metals. This production route thus limits the choice of metals which can be used.
An alternative method of producing the bi-metal tube is by explosive bonding. This method is implemented by placing a corrosion resistant tube within a steel tube and centralizing the two tubes. The outside and inside diameters of the inside and outside tubes respectively are dimensioned such that, on centralizing the two tubes, an annular gap is produced. From this point the method continues via one of two methods, i.e. expansion or implosion.
These methods are outlined in UK Patent Nos. 2,209,978 and 2,209,979 and in "A Fabrication Process for the Production of a Zirconius Bimetal Tube for Cl.sub.2 and H.sub.2 S Gas Wells", by R. Hardwick and C. T. Wang, in the Proceedings of the High Energy Rate Forming Conference, 1984, pp 189-194.
UK 2,209,978 relates to a method of forming bi-metal tubing by explosive bonding by expansion of a tubular component into engagement with a surrounding metal component. In this method an explosive charge is disposed axially and fired within a shock wave-transmitting insert located within the portion of the tubular metal component to be expanded, the shock wave-transmitting insert comprising a hollow cylindrical container fitting closely within the portion of the tubular metal component to be expanded and having charge-holding means to accommodate the explosive charge and locate it axially within the container, the container being filled with a shock wave-transmitting liquid.
UK 2,209,979 describes a method of forming bi-metal tubing as described in UK 2,209,978 wherein the outer tube is supported by a metal die member, surrounded by liquid.
Hardwick and Wang describes a process of producing zirconium/steel bi-metal tubes by explosive bonding by implosion, using an external annular charge, of a steel outer tube directly, and without an interlayer, to a zirconium inner tube to form a bonded shell. The bonded shell is then co-extruded within a hollow steel billet to form an extruded shell of zirconium lined steel of the requisite wall thickness proportions. The extruded shell is then further processed by a tube reducer, to the final tube size.
Hardwick and Wang also outline the process of producing bi-metal tubes by explosion, rather than implosion.
Material compatibility is not a limitation of explosive bonding as is the case in the method of co-extrusion. There are, however, limitations to the methods of explosive bonding by expansion or implosion disclosed in the prior art referred to above.
In the method of expansion, for example, the inner tube bore defines the volume of explosive which can be contained within it. If the wall thickness of the inner tube is sufficiently thick, a situation will arise where the tube bore cannot contain sufficient explosive to achieve bonding. This therefore defines a relationship between the tube bore, the wall thickness, and the material used.
Further, the outer tube wall thickness should be sufficiently thick if the outer tube itself is not to be expanded by the explosive charge. Should expansion of the outer tube occur, not only is dimensional control lost but the collision pressure occurring at the interface between the tubes is reduced, so leading to reduced bond quality. This problem may be overcome by use of an external die, as suggested in GB A 2,209,979. However this solution is time consuming, labor intensive and expensive.
A further disadvantage of the expansion method is that the detonation rate of the explosive is accelerated by the progressive increase in pressure within the tube bore. A situation may therefore occur where the detonation rate increases to a point beyond the upper limit for bonding. Consequently the length of tube which may be bonded by the expansion method is limited.
The method of implosion also suffers from a number of disadvantages. For example, the wall thickness of the outer tube being imploded is limited.
Further, in order to ensure bonding, the interfacial annular gap should be a minimum of around 20% of the outer wall thickness. Thus, if the outer wall is thick the gap will be substantial, and a situation will arise where the degree of contraction required of the outer tube is excessive. Surface wrinkling may therefore occur to depths which will not be removed by the bonding process, the control of such wrinkling being essential to the explosive bonding process.
A further disadvantage of the implosion method previously used is that the upper limit of tube length which can practically be achieved is 3-4 meters. This is due to the difficulty in attaining a uniform explosive density along the length of the annular charge. Variations in explosive density may affect the detonation velocity and so cause the detonation front passing down the annular gap to be destabilized, and increasingly distorted as a function of distance. This continues until the associated collision front at the interface below the detonation front is no longer travelling exclusively in a longitudinal direction but also circumferentially in opposing directions. When these opposing fronts meet at a diametrically opposite point, adiabatic compression of air in front of the collision front causes excessive melting of the surface preventing metal-to-metal bonding and also causing potential rupture of the inner tube.
Both the expansion and implosion methods are relatively expensive as they are extremely labor intensive. Further, the length limitations mean tube lengths are short, thus resulting in a high frequency of joints in an extended pipeline.
The explosion method disclosed by Hardwick and Wang overcomes some of the above problems by cladding of a steel layer onto the outer surface of a zirconium tube to form a composite shell, the shell then being placed in a steel bolster billet and hot co-extruded. This method enables lengths of tubing each typically of 12 meters in length to be produced from a single explosion, thereby increasing the total length of tubing available from a single explosion by a factor of 12 over previous explosive methods.